A bandgap reference circuit provides a stable output reference voltage, and is typically used in large integrated circuits for applications such as in telecommunications. It is desirable for the output reference voltage to remain stable with respect to temperature, manufacturing process variations, and in the case of a bandgap reference circuit to provide a continuous output reference voltage. The output reference voltage provided by known bandgap circuits, however, typically varies somewhat with respect to one or more of these factors.
A basic bandgap reference circuit 10 known in the art is shown in FIG. 1, and further described by Ahuja, B. et al., "A Programmable CMOS Dual Channel Interface Processor for Telecommunications Applications", IEEE Journal of Solid State Circuits, vol. SC-19, no. 6, Dec. 1984. Bandgap reference circuit 10 generally comprises an output circuit 20 and an operational amplifier 30. Output circuit 20 comprises a resistor 21, a resistor 22, a resistor 23, a bipolar transistor 25, and a bipolar transistor 26. Operational amplifier 30 comprises an ideal operational amplifier 32, and an offset voltage source 34. Ideal operational amplifier 32 has a positive input terminal, a negative input terminal, and an output terminal providing a bandgap reference voltage signal V.sub.BG. Offset voltage source 34 has a positive terminal, and a negative terminal connected to the negative input terminal of ideal operational amplifier 32. Resistor 21 has a first terminal connected to the output of ideal operational amplifier 32, and a second terminal connected to the positive input terminal of ideal operational amplifier 32. Resistor 22 has a first terminal connected to the output of ideal operational amplifier 32, and a second terminal connected to the positive terminal of offset voltage source 34. Resistor 23 has a first terminal connected to the positive terminal of offset voltage source 34, and a second terminal. Transistor 25 has an emitter connected to the second terminal of resistor 21, a base connected to a negative power supply voltage terminal V.sub.SS, typically zero volts, and a collector connected V.sub.SS. Transistor 26 has an emitter connected to the second terminal of resistor 23, a base connected to V.sub.SS, and a collector connected to V.sub.SS.
Bandgap reference circuit 10 provides output reference voltage V.sub.BG, whose value depends on the sizes of the components in a feedback loop of output circuit 20 between the positive input terminal and the negative input terminal of ideal operational amplifier 32. The value of V.sub.BG can be determined because an ideal operational amplifier changes its output until a voltage on the positive input terminal equals a voltage on the negative input terminal, in accordance with the following equation: EQU V.sub.BG =V.sub.BE1 +(R1/R2).DELTA.V.sub.BE +(1+R1/R2)V.sub.OS
where V.sub.BE1 represents the base-to-emitter voltage drop of transistor 25, V.sub.BE2 represents the base-to-emitter voltage drop on transistor 26, .DELTA.V.sub.BE represents the difference in base-to-emitter voltages between transistor 25, V.sub.BE1, and transistor 26, V.sub.BE2, V.sub.OS represents the voltage provided by offset voltage source 34, R1 represents the resistance of either resistor 21 or resistor 22, and R2 represents the resistance of resistor 23. As the performance of the operational amplifier improves and approaches that of an ideal operational amplifier, V.sub.OS approaches zero volts.
The base-to-emitter voltage of a bipolar transistor, labelled in general V.sub.BE, decreases as temperature increases. On the other hand, .DELTA.V.sub.BE rises with respect to temperature. Therefore R1 and R2 can be chosen to compensate for this variability with respect to temperature, such that as V.sub.BE1 rises, (R1/R2).DELTA.V.sub.BE falls in proportion, keeping V.sub.BG unaffected. However V.sub.OS introduces a component to V.sub.BG for which the values of R1 and R2 cannot compensate. If CMOS technology is used, V.sub.OS is typically from 10 to 20 millivolts, and varies with temperature, so that bandgap reference circuit 10 provides a relatively unstable output reference voltage. Several methods have been used to improve the output reference voltage of the basic bandgap reference circuit. One method is disclosed by Ulmer and Whatley in U.S. Pat. No. 4,375,595. This method relies on a technique in which the output reference voltage of the bandgap reference circuit is not always available, however, and so cannot be used for applications requiring a continuous output reference voltage.
In time-continuous output reference voltage applications, a known method to lower variability of the output reference voltage with variations in temperature, due to the effect of the offset voltage, is to utilize an area ratioed stack of bipolar transistors in the output circuit to provide the feedback loop, as disclosed in the previously mentioned Ahuja reference. The area ratioed stack approach utilizes a larger feedback loop than output circuit 20 of bandgap reference circuit 10. Instead of a single transistor, for example transistor 25, connected to an input of the operational amplifier, two or more transistors are used in a chained fashion, wherein the emitter of a transistor is connected to the base of the next transistor of the chain. The contribution of the error term, defined as (1+R1/R2)V.sub.OS, is a smaller fraction of V.sub.BG, because the absolute value of V.sub.BG is increased.
In CMOS processing, bipolar transistors which have stable threshold characteristics well suited for use in bandgap reference circuits can be fabricated in either of two modes. See, for example, Degrauwe, M., et al., "CMOS Voltage References Using Lateral Bipolar Transistors," IEEE Journal of Solid Stae Circuits, vol SC-20, no. 6, Dec. 1985. In a vertical implementation, a bipolar transistor is formed by a diffusion, a well, and a substrate forming an emitter, a base, and a collector. The vertical bipolar transistor is limited in that the collector, being formed in the substrate, is typically tied to a power supply terminal. In a lateral implementation, a bipolar transistor is formed by a first diffusion, a well, a second diffusion, a substrate, and a gate, as disclosed by Degrauwe et al. In the lateral implementation, a free collector is available, but a proportion of an emitter current flowing out of the free collector varies widely, from about 30% to 70%. Use of lateral bipolar transistors as input transistors of an operational amplifier with low offset voltage is taught by Rybicki et al. in U.S. patent application, Ser. No. 358,980 filed 5-30-89, entitled "An Operational Amplifier with Reduced Offset Voltage Using Lateral Bipolar Transistors." However the existence and variability in currents of the lateral bipolar transistors in the input stage of the operational amplifier can create errors in applications such as bandgap reference circuit 10.